wiring a dvc sub and performance

i have one rockford fosgate 12" HE2 dvc sub 4ohm x 2, a rockford fosgate 401s amp, and a R&T Enterprises 12" Single Bandpass Subwoofer Box. i bought all this in best buy since i work there. my amp only allows a minum of 4ohm bridge load; but is it safe to go down to 2ohm. how should i wire my sub because i dont know if i wire it at 8ohm would it decraese my power or is possible to wire my sub to 4ohms. how can i figure out which frequency to use on my amp crossover and how to tune the box 2 ports to get good sound.

sorry for the long question but i might ask more if i get more question to my head.

Your sub and amp are not a very good match for one another. For optimal performance I would suggest that you exchange one or the other for a better match. As far as bridging the amp to 2 ohms, I'm not suggesting that you do it, but I have bridged other 4 ohm stable Fosgate's to 2 ohm without problems ( DO AT YOUR OWN RISK) . I do not believe that your enclosure can be tuned, I It comes from the factory at a set frequency. If you don’t want to bridge it to 1X 2 ohms then your only other option would be to wire both coils (bridged) to one of the 2 channel on you amp for a 2 ohm load on that channel which will give you 200 watts. 

That amp is rated at 2X100 watts RMS @ 4 ohms.  The best you could do is wire the sub with one VC to each channel and make SURE your inputs to each channel are the same (not the safest way) or as bberman suggested, parallel the VCs at 2 ohms to ONE channel.  You cannot use that sub bridged at 2 ohms to that amp without risking overdriving the amp,.  To run it bridged you'd have to wire it at 8 ohms which still nets you about 200 watts from the 401s  Your best bet would be to upgrade to a 2-ohm stable mono amp (like the Power 501bd, if you want to stay with Rockford Fosgate equipment) or get a sub that is 2X2 ohms or 2X8 ohms... or 1X4 ohms SVC!

I don't know what the tuning frequency of that enclosure is, but I suspect it is around 50 to 55 Hz.  So your crossover should be set at, say, 80 to 100Hz to take advantage of the full bandpass curve.  After you setup your system, try it and see what sounds best.

Still would not recommend it, if the amp is still new, the time to swap is now. If not well only time will tell, warranty expires at some point in time and usually the problem happens right after warranty expires. Can you damage the sub, yes definitely.